The present invention relates to a method for preventing bleeding of plasticizers on the surface of shaped articles of a plasticized polyvinyl chloride resin.
Polyvinyl chloride resins are one of the most important classes of thermoplastic synthetic resins used for manufacturing various kinds of shaped articles useful in a very wide field of applications. Shaped articles of polyvinyl chloride resins are classified into rigid type ones and flexible type ones according to the absence and presence of a substantial amount of a plasticizer in the resin compositions. Namely, the rigidity or flexibility of shaped articles of a polyvinyl chloride resin can be controlled by the incorporation of a plasticizer such as a phthalic ester and the thus plasticized polyvinyl chloride resins are fabricated into various kinds of flexible articles including films, sheets, synthetic leathers, tubes, hoses, bags, packings, covering materials and the like directed to the uses in the fields of medical and sanitary wares, cooking wares, wrapping materials for foodstuffs, insulation of electric wires, materials used in agricultural plant growing, materials for buildings and the like.
One of the most difficult problems involved in the shaped articles of plasticized polyvinyl chloride resins is the so-called bleeding of the plasticizer contained therein which is a phenomenon that the plasticizer contained in the body of the shaped article migrates with time toward the surface of the article and is lost by evaporation into the atmosphere. This phenomenon of bleeding of the plasticizer as well as of the other additives contained in the shaped articles is so remarkable that not only the beautifulness in appearance but also various mechanical or physical properties are detrimentally affected giving rise to a shortened serviceable life of the articles.
The bleeding phenomenon is especially undesirable when the shaped article is used in a medical application such as a bag or container of blood for transfusion or used in contact with foodstuffs because the toxicity of the plasticizer and other additives may thereby limit the usage of polyvinyl chloride resins.
Various attempts have been made to decrease the bleeding of plasticizers and other additives on the surface of articles including techniques utilizing the irradiation with ionizing radiations or ultraviolet light, corona discharge at a relatively high gas pressure, e.g. larger than 100 Torr, and treatment with chemicals. These prior methods are to some extent effective for improving various surface properties of articles of polyvinyl chloride resins such as heat resistance, anti-solvent resistance, affinity with water, electrostatic charging, printability and the like but the effectivness of these method in preventing plasticizer bleeding is rather small. Moreover, it is sometimes unavoidable that certain desirable properties of the shaped articles of polyvinyl chloride resins are adversely affected by these methods.
For example, the irradiation with an ionizing radiation produces crosslinks not only in the surface layer but also in the body of the article and its attendant high energy may result in undesirable change in the mechanical properties of the plasticized polyvinyl chloride resins. The irradiation with ultraviolet light adversely affects the coloring of the surface due to the oxidative degradation taking place in the surface layer of the articles. The treatment with corona discharge is not practicable due to the large difficulty in the process control owing to the rather unstable nature of the discharge. Further, chemical means are not free from the problems of erosion by the chemicals or poor adhesiveness or durability of coating compositions.